1. Field of the Invention
The present invention relates to level sensing and, more particularly, to electro-optical devices for measuring the distance between a reference level and a variable surface, such as the surface level of liquid in a vessel.
2. Description of the Prior Art
There are various techniques for determining the level of a liquid in a vessel. One category of techniques involves physically measuring the level, such as with a dip stick, a float indicator or the like, but such intrusive techniques entail a number of disadvantages. They occupy space in the vessel, they contact the liquid in the vessel with the risk of either contaminating it or being contaminated by it, and they may require periodic access to the interior of the vessel. This requires exposing the interior of the vessel to atmospheric pressure and may violate government regulations which prevent venting of the vessel to atmosphere.
Accordingly, there have been developed a number of nonintrusive level gauging techniques. One such technique involves the use of a light beam which is directed to and reflected from the surface of the liquid in the vessel. Such optical level measurement systems typically utilize the angle of reflection of the incident light beam from the surface of the liquid to indicate the distance of the surface from a reference level. Typically the difference in incidence and reflection angles between full and empty vessels is very small and, therefore, accurate level measurement requires very precise mounting of the optical equipment; which can entail significant expense.